Oct. n, 1915 
Phytophthora infestans in Irish Potato 
81 
through the winter. The tubers selected showed various stages of 
infection; but none were used that did not show at least one living eye 
(bud). On June 6 the tubers were planted in a 2-acre field of potatoes 
somewhat isolated from adjoining fields, 256 being planted whole in two 
rows 8 rods long. In a third row 162 hills were planted with cut infected 
seed. Alternating with these, three rows were planted with healthy 
seed, Green Mountain variety, as checks. The seed was planted between 
1 and 2 inches deep and the row hilled up so as to cover the sets from 
3 to 5 inches. Continuous records were taken of the soil temperature 
by means of a self-registering Richard soil thermograph. A record was 
also kept of the rainfall, especially as to the date and approximate 
amount. 
As would naturally be expected, the infected whole tubers sent up 
shoots more rapidly than the cut seed. Six of the whole tubers had 
shoots through the ground two weeks after they were planted. On 
July 6, 30 days after planting, 63 per cent of the whole infected tubers 
had shoots up; so also did 49 per cent of the cut infected seed and 97 
per cent of the tubers planted in the three control rows. After July 
6 the percentage increased very little in any of the foregoing cases. 
On this same date six of the whole diseased tubers that had failed 
to send up shoots were dug up for examination. Tour of these were 
dead and nearly decayed, while the remaining two had two and 
five shoots, respectively, which were just ready to break through the 
surface of the soil. Plate VI, figure 2, shows the condition of one of 
these shoots immediately after digging. They were taken to the lab¬ 
oratory later and examined for spores of P. infestans , but none were 
found. Subsequently they were placed in a moist chamber overnight, 
and the next morning small patches of conidiophores bearing spores, 
which on microscopic examination proved to be those of P. infestans , 
were found scattered over the diseased areas. The infected shoots were 
very much like those obtained in the laboratory experiments discussed 
earlier. It should be noted that a few days before the plants were dug 
up a light shower of rain had fallen, which, it is believed, materially aided 
the progress of the fungus. These developments in the field experiments 
are wholly comparable with those in the laboratory, in which the sprouts 
were attacked and overrun by the disease before reaching the surface of 
the soil. 
On July 13 a very interesting case developed in the row planted with 
infected cut seed. When the infection was first noted, the discoloration 
had extended up the stem of the plant only half an inch above the surface 
of the soil. There was no evidence of spores of P. infestans. The w;eather 
was clear and the humidity unusually low, a condition not favorable for 
sporulation of P. infestans . The plant was carefully watched the fol¬ 
lowing day, but no evidence of sporulation could be detected. The next 
